The Subversion plugin for OnTIme assumes that you're using optimistic locking for your SCM tool, and does not support exclusive locking of files (which is what the checkout button in OnTime is designed for).
It seems like a bug to me that the check out button is enabled for the subversion plugin, I'll have to take a look at that. For now, you can just edit the file locally, and then use OnTime to do your commits through subversion.
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